In general, a composite material may be defined as any material containing a reinforcement material which is supported by a binder material. Composite materials thus comprise a two-phase material having a discontinuous reinforcement material phase that is stiffer and/or stronger than the continuous binder (matrix) phase.
Composite materials having a resin binder phase offer substantial advantages over other materials such as metals, alloys and wood. Parts made from composites are thus often much lighter than parts made from steel or other materials, producing tremendous advantages for such composites in terms of the strength to weight ratios. Also, such composites often offer significant advantages over other materials as regards chemical and corrosion resistance and superior physical performance properties such as, for example, tensile properties.
Because of the advantages that may be derived from using composites, such materials have been proposed, and used, for a variety of end use applications ranging from various household products (e.g., bathtubs and shower enclosures) to the transportation industry (e.g., boat hulls and structural components of automobiles). Many of such end use applications require that the part be thick, i.e., at least one-half centimeter in thickness. Parts that can be produced in thicknesses of 0.1 cm or more will satisfy most applications.
Several different fabricating techniques have been developed that may be used to produce composites. One such fabricating technique is resin transfer molding (RTM). RTM may be described, in general, as a process which uses a mechanical pumping apparatus to transfer catalyst and a reactive resin from holding tanks into a closed mold containing a reinforcement material. A variety of reinforcement materials have been used as have various resins, including, for example, unsaturated polyesters, epoxies and vinyl esters. Often, to allow the reinforcement to fit into the mold quickly, the reinforcement is shaped into the desired geometry in a separate operation. The use of such preforms can result in faster cycle times because this eliminates the need for time-consuming placement of the fibers or other reinforcement material at the production press where the composite part is actually fabricated. In addition, these preforms allow for precise control of the fiber placement.
Fabricating composites using RTM has widespread potential because this technology can draw upon the vast amount of technology developed over many years for reaction injection molding (RIM) techniques. Thus, RIM fabrication has been in widespread use for a variety of commercial applications over many years.
However, while RTM has already found extensive application for the low volume production of a variety of specialty products, further developments are required to develop suitable high volume production techniques using RTM. The two fundamental requirements of high volume or mass production are low cost and high speed.
Because the resins actually begin to cure before they enter the mold in RTM fabricating techniques, such resins must meet some rather stringent requirements. The curing resin should exhibit a moderate viscosity plateau while it is still flowing into the mold, but should cure rapidly once the mold is filled.
The fact that the resins in RTM techniques begin to cure before they enter the mold make meeting the requirements of low cost and high speed very difficult. One traditional method to attempt to increase the speed of RTM is to operate the system with increased mold pressures to accommodate more rapid curing rates. Unfortunately, however, the increased operating pressure seems to inevitably result in increased costs due to the more expensive molds and pumping systems that are required to achieve such rapid curing rates.
In addition, there are many technical problems in RTM which arise in the step of filling the mold and impregnating the reinforcement material, such as a preform. The mold filling step can become very complicated since a reacting liquid is being forced through a porous medium (i.e., the preform or other reinforcement material). As this liquid reacts, it becomes more viscous (actually viscoelastic). Indeed, thermoset systems typically exhibit a tremendous increase in viscosity as such systems cure due to branching and cross linking in the system. Due to the moderately high initial (uncured) resin viscosities that increase during reaction, most current RTM processes exhibit significant mold filling problems associated with high operating pressures required to fill the mold and poor resin impregnation into the preform or other reinforcement material.
Even further, displacement and/or compression of the reinforcement material may often occur as the curing resin flows into the mold. Such displacement and/or compression can undermine a major advantage of RTM which is the precise control over the reinforcement material placement. A still further problem which can occur that results in considerable delay and downtime is the gelation of the resin in the transfer lines before entry into the mold. In addition to the problems previously described, further problems in utilizing RTM fabricating methods may arise because the process is typically not isothermal and the mold geometry of the part being fabricated may be highly irregular.
Still further, it has been found that it is important to differentiate flow on the microscale (i.e., within the reinforcement material itself such as a fiber bundle) from flow on a macroscale (e.g., between fiber bundles). It has thus been suggested that microflow improves the wetting and bonding at the reinforcement material (e.g., fiber)/resin matrix interface, and therefore improves the strength of the final composite. The time required for microflow depends upon the viscosity of the penetrating fluid (the reacting resin in RTM), but may be on the order of hours for large parts using commercial resins. RTM has also traditionally been used for encapsulation of semiconductor and other microelectronic devices. U.S. Pat. No. 5,331,205; U.S. Pat. No. 5,344,600; Runyan, W. R. et al., Semiconductor Integrated Circuit Packaging, Addison Wesley, Reading, Mass. (1990); Licari, J. J. et al., Handbook of Polymer Coatings for Electronics, Noyes Publication, Park Ridge, N.J. (1990); Goosey, M. T., Plastics for Electronics, Elsevier, New York, N.Y. (1985); Matisoff, B. S., Handbook of Electronics Packaging Design and Engineering, Van Nostrand, New York, N.Y. (1990); Flick, E. W., Adhesives, Sealants and Coatings for the Electronics Industry, Noyes Publication, Park Ridge, N.J. (1986); and Buchanan, R. C., Ceramic Materials for Electronics, Marcel Dekker, New York, N.Y. (1991). Microelectronic devices are typically encapsulated in a protective thermoset body from which a number of leads extend to allow electrical contact and interconnection between the encapsulated semiconductor device and a printed circuit board. A consequence of the increasingly large number of features on a chip is the use of smaller and finer wires to electrically connect the semiconductor chip to a substrate. These extremely fine wires, which carry electrical signals to the chip, may be easily displaced (wire sweep) or damaged during encapsulation of the chip. If this happens, the device cannot be repaired and must be discarded.
Wire sweep has always been unavoidable during molding, but controlling it is an established challenge in the production of semiconductor devices. Certain factors contribute to the overall difficulty in limiting wire sweep. As stated previously, flowing molding compound exerts a drag force on the wires. If this force exceeds the strength of the wires or of the bonds, then the wires will bend in the direction of the force. Longer wires tend to sweep more easily than shorter wires; therefore, it is desirable to keep the wire lengths as short as possible. However, it is not always possible to keep the wire lengths short. Other constraints in the packaging technology are pushing the wires to longer lengths. It is often necessary to place a small die onto a large die pad or flag; that is, there is more than 0.64 mm clearance, a typical maximum constraint for this dimension, from an edge of the die to the corresponding edge of the flag. Having a die on a flag that exceeds the typical maximum allowable clearance often forces the connecting wires to be longer than desired. There is also a greater risk of sagging wires which would cause shorting in the device if the wires touch the metal flag. Furthermore, some packages are becoming larger in size in addition to having more pin counts. The QFP's (Quad Flat Packages) range in size from 7 mm.times.7 mm to 40 mm.times.40 mm. Larger package sizes generally correlate to longer wire lengths because some of the semiconductor die that are placed within these high lead count packages are much smaller than the smallest flag size that can be designed into the leadframe.
Another factor that contributes to the difficulty of controlling wire sweep is the proximity of the wires to each other. The closer the wires are together, the more critical it becomes to limit the wire sweep to reduce the possibility of wires coming into contact with each other. Miniaturization of the geometry of circuit patterns on a semiconductor die is resulting in bonding pads being designed closer together. Moreover, die designers are putting more components on a single die to expands its functions. Increased functionality of each chip results in more I/O's. More output pads and smaller die circuit geometry combine to make the packaging process more difficult because the wires get longer and also placed closer together, both on the semiconductor die and on the leadframe. Increased pin counts force the lead tips on the leadframe to be designed closer together. Some of the QFP's are already in production at 0.4 mm pitch between the leads and some are progressing toward 0.3 mm pitch and smaller.
An additional development in the packaging field that will contribute to the wire sweep problem is the emergence of fine pitched QFP (quad flat package) in molded carrier rings (MCR). The MCR poses a manufacturing problem because the molding process is more complicated than molding non-MCR packages. The MCR and the package can be filled sequentially with the MCR usually being filled first, or they can also be filled at the same time using a different gating configuration. The process window for this operation is very restrictive because the molding compound must be transferred quickly enough to fill both the MCR and the package before the compounds gels but it must also be transferred slowly enough not to cause excessive wire sweep. Because of the tight process window with this type of package, production yield can be affected since any deviation outside the established process window can cause molding rejects due to excessive wire sweep or incomplete filling of the part.
The process of chip encapsulation is important since the chip has already undergone many (perhaps hundreds) processing steps and is just one step away from being a finished product (and thus has considerable value which is lost if the encapsulation step fails). With the current encapsulant transfer molding process, it is very difficult to control the problem of wire sweep in a high pin count, fine pitch package. New trends in die design and packaging pose an increasing challenge to the wire sweep problem. Indeed, due to the continuing advances in device capabilities and rapid changes in circuit board assembly methods, the packaging step is more important than ever before, and may, for the first time, impose limits on the design and performance of the final semiconductor device. Manzione, L. T., Plastic Packaging of Microeletronic Devices, Van Nostrand Reinhold, New York, N.Y. (1990).
As indicated, most transfer molding processes suffer from significant problems such as high operating temperatures and pressures required to fill the mold, and poor resin impregnation. Using higher temperatures results in faster cures but often increases the problem of wire sweep. Therefore, current transfer molding processes are plagued by two important problems: i) wire sweep due to the high initial melt viscosity, and ii) inadequate time to fill the mold due to rapid cure (concurrent with mold-filling) at the elevated operating temperatures. As the package sizes and the associated wires shrink to smaller and smaller dimensions, so does the operating window for transfer molding; hence, it is difficult to control wire sweep in the high pin count, fine pitch packages. These problems lead to decreases in productivity due to increased packaging-related rejects in the final processing step, for these new fine pitch microelectronic devices. In addition to the above-mentioned processing limitations, transfer molding operations require expensive molds, which discourages experimentation with new circuit designs and layouts.
In summary, limitations in the use of RTM in high volume production arise from the fact that the resin systems used begin to react before the system enters the mold. This initial reaction thereby creates relatively high viscosities, requires high operating pressures, and results in poor wet-out of the reinforcement material as well as displacement thereof. Additionally, attempts to decrease the cycle time by increasing the operating pressure to accommodate faster reactions may actually exacerbate the problem by decreasing the quality of the composite by affording insufficient time for microflow. Finally, the high capital costs associated with high operating pressures create an inevitable trade-off between high speed and low cost under current RTM technology.
Another technique that may be used for producing thick and complex parts from composites or other materials is hand layup. While quite acceptable for customized applications, this method is obviously extremely labor intensive and offers no potential whatever for high volume or mass production.
Photopolymerizable compositions and technology for using such compositions has been known for many years and has been proposed for a wide variety of applications. As one illustrative example, it has been proposed to utilize certain photopolymerizable compositions including various fillers to form dental compositions. U.S. Pat. Nos. 4,762,863, 4,933,376 and 4,977,197 to Sasaki et al. are patents disclosing suitable photopolymerizable compositions and reinforcing fillers.
Utilizing photopolymerizable compositions to fabricate a thick part can be difficult. Thus, with relatively thick parts, light intensity gradients will typically result, and such gradients can prevent satisfactory curing throughout the thickness of such parts. More particularly, what can often occur is the polymerization of thin layers adjacent to the surface where the light source is positioned. The thickness of this layer is determined by the distance the initiating light may effectively penetrate. Perhaps for this reason, the use of photopolymerizable compositions has been largely directed to forming thin films or coatings. U.S. Pat. No. 5,340,653 to Noren et al. is one example of a free-radical curable composition used as a coating for various substrates.
One possible exception to the use of photopolymerizable compositions for thin films and coatings is U.S. Pat. No. 5,137,800 to Neckers et al. This Neckers et al. patent concerns forming three-dimensional objects by stereolithography using the general method described in U.S. Pat. No. 4,575,330 to Hull. As is disclosed in the '800 patent, a photopolymerizable monomer and a photoinitiator system for the monomer is used in a method that involves directing a ray of activating radiation for the photoinitiator system to and through a point in a given plane and into the body of the composition. The intensity of the activating radiation of the ray or the time during which the ray entering the body is directed through the point is employed to determine the distance through which the ray of activating radiation enters the body to the point of the succession of points to which the ray activates the photoinitiator system that is farthest from the surface.
More particularly, when the three-dimensional polymerizing method is practiced, using a visible light photoinitiator, such as eosin and its derivatives and visible light for activation, it is stated that it is not necessary that the photoinitiator have a peak absorbance at the wavelength of the activating light. All that is required is that there be sufficient absorbance at the wavelength of the activating light to cause the reactions which form the activator and cause the dye to lose its color at the required rates. As is noted, if bleaching occurs too rapidly, by comparison with the rate at which polymer-forming radical formation occurs, radical reactions which do not cause polymerization, e.g., radical coupling, can be expected. It is stated that, if polymerization occurs too rapidly, by comparison with the rate at which bleaching occurs, activating light cannot penetrate the monomer/photoinitiator mixture to a sufficient depth, and polymerization stops on or near the surface through the light enters. Referring to the foregoing working Examples in the '800 patent, Neckers et al. state that the balance was achieved by the selection of certain dyes as photoinitiators, using triethanolamine as an accelerator, and controlling the proportions of the two. In general, it is stated that the photoinitiators are used at extremely low concentrations by comparison with those which have previously been suggested and used, most frequently in curing thin films. On the other hand, it is noted that the concentration of the photoinitiator must be sufficiently high that the induction period is not excessive, and the concentration of the activator and the photon density must both be sufficiently high that the rates of bleaching and polymerization are appropriately matched to achieve polymerization to a desired depth in a photopolymerizable composition. (col. 22, II. 19-58).
This Neckers et al. patent uses a highly focused light source. Thus, certain lasers were used in the methods disclosed in the working Examples. Also, in describing the reaction using a system including trimethylopropane triacrylate with eosin lactone and triethanolamine, and using a beam of visible light from an argon ion laser having a wavelength of 514 nm, Neckers et al. state that the eosin undergoes a reaction with the triethanolamine, producing two moieties, one of which is a free radical which serves as an activator for the polymerization of the trimethylopropane triacrylate. The loss of the dye color also occurs as a consequence of the eosin reaction with the triethanolamine, enabling subsequent radiation from the argon ion laser to penetrate farther into the composition and to cause reaction of eosin with triethanolamine and activation of the trimethylopropane triacrylate at the level of greater penetration, and still greater penetration by subsequent radiation with consequent reaction and activation at the level of the still greater penetration. (col. 2, I. 58 to col. 3, I. 7).
As may be appreciated, producing materials by stereolithography involves rastering a laser beam across the surface of a pool of monomer to form a thin polymeric layer. This polymerized layer is then lowered slightly into the pool of liquid monomer, and fresh monomer flows into its place at the free surface. A second thin layer of polymer is then formed at the free surface, and this second layer adheres to the layer below it. By repeating this process, it is possible to make thick polymeric parts. While stereolithography perhaps may be suitable for some applications such as developing a prototype, the slow process rates make stereolithography unsuitable for large-scale production of polymeric or composite parts.
Despite all of the considerable work in this field, there exists the need for a fabricating method for thick polymeric or composite parts and the like which is amenable to high speed, low cost production. To this end, a principal object of the present invention is to provide a composition suitable for making thick polymeric or composite parts that is amenable to high speed, low cost production and is energy efficient.
A further object of this invention is to provide polymeric or composite parts capable of being made from a variety of monomer/resin systems so as to allow tailoring of the properties to the particular requirements of the end use application.
A still further object is to provide a reactive composition for making thick composites which is characterized by a relatively long shelf life.
Yet another object of the present invention lies in the provision of a facile process which may be utilized to form thick parts by widely varying techniques.
A further object of this invention is to provide an improved process for forming composites utilizing RTM technology and achieving high speed, low cost production capabilities.
A still further object is to provide a method for making polymeric or composite parts which allows decoupling of the mold filling operation from the initiation of the reaction itself together with the attendant advantages thereby achieved. A more specific aspect thus provides an RTM process characterized by the ability to utilize low viscosity monomers, allowing rapid mold filling at low pressures, efficient penetration of the reactive liquid into the reinforcing material without displacement thereof, and the like.
Yet another more specific aspect lies in the elimination of gelation in the respective transfer lines used in a molding system so as to facilitate automation.
There also exists a need for a method for encapsulating microelectronic devices such as semiconductors and capacitors that reduces or eliminates wire sweep and damage to the microelectronic device as well as encouraging development of new circuit designs and layouts while reducing the cost. Thus, a principal object of the present invention is to provide a composite suitable for encapsulating microelectronic devices.
A further object of this invention is to provide a method for encapsulating microelectronic devices that decreases or eliminates damage due to wire sweep during encapsulation.
A still further object of the present invention is to provide a cost-effective method for encapsulating microelectronic devices by reducing production costs and increasing process yields.
Yet another object is to provide a method for encapsulating microelectronic devices that is flexible enough to encourage new circuit designs and layouts by eliminating the need for expensive molds required for RTM processes.
Other objects and advantages will become apparent as the following description proceeds.